1. Field of the Invention
The invention relates to a circuit arrangement for igniting a high-pressure discharge lamp, provided with
- input terminals for connection to a commutating supply source with a square-wave supply voltage; PA1 - a pulse-generating circuit provided with a voltage-dependent breakdown element and with a voltage-raising network formed by rectifying and capacitive means; PA1 - a pulse transformer; and PA1 - an electrical connection between a secondary winding of the pulse transformer and lamp connection terminals.
2. Description of the Related Art
A circuit arrangement of the kind mentioned in the opening paragraph is known from EP-A-0 398 432 (U.S. Pat. No. 5,087,859). The known circuit arrangement forms part of a circuit for operating and igniting a high-pressure discharge lamp as part of a projection TV system. Special requirements as to dimensions and luminous efficacy are imposed of a lamp if it is to be suitable for such an application. This results in that an ignition pulse of more than 10 kV, often even more than 20 kV, is required for reliable ignition of the lamp. A further requirement imposed on the ignition circuit is that an extinguished lamp which has not or hardly cooled down must be capable of quick re-ignition. Such hot re-ignition requires a re-ignition pulse of a level corresponding to that of the ignition pulse.
In the known circuit arrangement, ignition pulse generation takes place through excitation of a tuned LC circuit by which the voltage across the capacitive means of the voltage-raising network is so increased that the breakdown voltage of the breakdown element is reached. The supply source voltage is commutated with a comparatively high frequency of a few tens of kHz to achieve an efficient operation during the process of ignition pulse generation. After the connected lamp has ignited and a stable arc discharge has developed, the commutation frequency of the supply voltage switches to a comparatively low value. By choosing a high commutation frequency in the ignition phase, it is indeed possible to use comparatively small components and the generation of a sufficiently high voltage will occupy comparatively little time. A disadvantage, however, is that the supply source circuit must be fitted with means for changing the commutation frequency in dependence on the state of the lamp.